1. Field of the Invention
The present invention relates to a sheet feeding apparatus, a sheet conveying apparatus, and an image reading apparatus for use in an image forming apparatus, such as, a copying machine, a printer, a facsimile machine, or other similar image forming apparatus, that feed and convey a sheet to an image reading position.
2. Discussion of the Background
An image forming apparatus, such as, a copying machine, a printer, a facsimile machine, or other similar image forming apparatus, uses various types of auto document feeding devices that feed original documents having images to be read by an image reading device to an image reading position, and various types of sheet conveying devices that convey original documents to an image processing device. There is an auto document feeding device or a sheet conveying device that has been especially widely used, in which several original documents out of a stack of original documents (hereafter “sheets”) are stacked on an original document setting table and picked up one by one by a sheet pick-up roller, and then conveyed by a sheet feeding belt and a reverse roller provided downstream of the sheet pick-up roller in the sheet conveying direction. Recently, in an auto document feeding device or a sheet conveying device that conveys original documents to an image reading position, various types of original documents have been used. For example, demands for copying a color copy sheet as an original document have increased.
In the above-described auto document feeding device of an image forming apparatus, for example, a stack of original documents are set on an original document setting table lying face-up so that a user can see the image of the original document. After the original document has been separated from the other original documents, and while it is conveyed to an image reading position, the original document is reversed. As a result, the image of the original document directs downward at the image reading position and is read by an image reading device located below the original document. The original documents are discharged from the auto document feeding device after their images have been read by the image reading device, and are sequentially stacked such that the front side of each page of the original documents directs downward. As a result, the original documents are stacked in a correct order of pages.
In the case of a color original document, silicone oil is often applied onto the surface of the color original document. When the silicone oil is adhered to a sheet pick-up roller and a sheet feeding belt, sheet feeding forces of the sheet pick-up roller and sheet feeding belt decrease as the number of sheets fed by the sheet pick-up roller and sheet feeding belt increases. As a result, problems, such as a sheet jam, typically occur. In the auto document feeding device or a sheet conveying device, a reverse roller is provided opposite a sheet feeding belt. When the sheet feeding belt and the reverse roller are driven, the sheet feeding belt and the reverse roller rotate in opposite directions. For example, when the sheet feeding belt feeds the first page of a stack of original documents toward the downstream side in the sheet feeding direction, the reverse roller obstructs the feeding of the second or following pages. Thus, because of the operations of the sheet feeding belt and the reverse roller, the original documents are fed one by one.
Because silicone oil is not adhered to the reverse roller, but is adhered to the sheet feeding belt and the sheet pick-up roller, the balance is lost between the reverse force (opposite the sheet moving direction) of the reverse roller and the forward forces of the sheet feeding belt and the sheet pick-up roller. Specifically, the reverse force of the reverse roller exceeds the sheet feeding forces of the sheet feeding belt and the sheet pick-up roller, thereby increasing the slip ratio of the sheets. Further, the slip of sheets may be caused by uneven application of silicone oil to the surface of an original document. For example, when a large amount of silicone oil is applied to the leading end portion of an original document, the original document may slip when being picked up by the sheet pick-up roller and may not slip after being picked up by the sheet pick-up roller. When a large amount of silicone oil is applied to the trailing end portion of an original document, the original document may slip after being picked up by the sheet pick-up roller.
In the above-described auto document feeding device or sheet conveying device, a pair of pull-out rollers (so-called sheet abutment rollers) are provided downstream of the sheet feeding belt in the sheet feeding direction. To reduce the size of the auto document feeding device or sheet conveying device, the devices are configured so that a drive motor is rotated in a forward direction for rotating the sheet pick-up roller and the sheet feeding belt, and the drive motor is rotated in a reverse direction for rotating a pull-out roller. In this configuration, an original document is picked up by the sheet pick-up roller and fed by the sheet feeding belt while the drive motor is rotated in the forward direction, and is then abutted against a nip part of the pull-out rollers in a halt condition while feeding the original document a distance greater than a sheet feeding path to perform a sheet skew correction. In the sheet skew correction, the leading edge of the original document is aligned, and thereby the posture and position of the original document are registered. The sheet feeding distance for abutting the original document against the nip part of the pull-out rollers is set based on information detected by a sensor provided immediately before the pull-out rollers. However, if the slip of the above-described color original document to which silicone oil is applied is not considered, the leading edge of the original document may not abut against the nip part of the pull-out rollers. In this case, a sheet feeding failure occurs at the pull-out rollers.
There is a background sheet feeding device that includes one sheet detecting device and a control device that controls the pulse number of a motor, which drives a sheet feeding roller. In this background sheet feeding device, the leading edge of a sheet fed by the sheet feeding roller passes the sheet detecting device, and abuts against a nip part of a pair of pull-out rollers in a halt condition. Subsequently, the sheet feeding roller rotates by a predetermined amount and a loop is formed on the sheet for a sheet skew correction. In this configuration, the control device controls a drive amount of the motor, after the leading edge of the sheet abuts against the nip part of the pull-out rollers, in accordance with the pulse number of the motor when the leading edge of the fed sheet passes the sheet detecting device. With this control of the drive amount of the motor, an adequate loop is formed on the sheet for the sheet skew correction.
However, in the above-described sheet feeding device, because the sheet feeding device uses only one sheet detecting device, the control device cannot determine at which point in the sheet feeding path between the sheet feeding roller and the pull-out rollers the sheet is delayed in its feeding. Further, the actual sheet feeding speed at the position adjacent to the pull-out rollers cannot be clearly determined. As a result, the drive amount of the motor necessary for performing a sheet skew correction may not be set with accuracy.